In recent years, as the range of application of a magnetic recording device, especially a magnetic disk drive, is extended, it is requested that the magnetic recording device be made smaller and have a larger capacity than before. In order to meet such request, it is essential to improve the recording density in a magnetic recording medium used in the device.
As a technique of increasing the linear recording density, a perpendicular magnetic recording method is widely used instead of a known longitudinal magnetic recording method. The perpendicular magnetic recording method is a method of forming a recording bit such that magnetization of a recording medium is performed in a direction perpendicular to a surface of the recording medium and magnetization of adjacent recording bits is performed in a direction antiparallel to each other. In the case of the perpendicular magnetic recording method is used, a demagnetizing field in a magnetization transition region is small. Accordingly, a steep magnetization transition region is formed and magnetization is stabilized with a high density compared with the longitudinal magnetic recording method. Therefore, as compared with the longitudinal magnetic recording method, it is possible to enlarge the magnetic particle volume by increasing the film thickness in order to obtain the same recording resolution and to suppress attenuation of recorded magnetization with the passage of time, that is, thermal demagnetization. In addition, since a high recording magnetic field is obtained in combination of a single magnetic pole head and a perpendicular magnetic recording medium including a perpendicular recording layer and a soft magnetic underlayer, it becomes possible to select a material having high anisotropy energy for the perpendicular recording layer. As a result, the thermal demagnetization can be further suppressed.
For a magnetic recording layer of a medium used in the perpendicular magnetic recording method, a granular structure in which a crystal grain is surrounded by a non-magnetic compound, such as an oxide or a nitride, has been proposed. For example, Japanese Patent Publication 2002-342908 (“Patent Document 1”) discloses a magnetic recording medium with a recording layer which has a Co—Cr—Pt alloy as a main material and contains an Si oxide, the content of Si being 8 at % or more and 16 at % or less in terms of Si atoms. Furthermore, “Role of Oxygen Incorporation in Co—Cr—Pt—Si—O Perpendicular Magnetic Recording Media”, IEEE Transactions on Magnetics, Vol. 40, No. 4. July 2004, pp. 2498-2500, (“Non-patent Document 1”) reports that a recording layer made of Co—Cr—Pt—Si—O is formed on a soft magnetic underlayer made of Co—Ta—Zr and having a thickness of 160 nm with a Ta/Ru intermediate layer interposed therebetween and a coercive force is maximized to improve an S/N ratio when the concentration of oxygen in the recording layer is about 15%.
An object of the techniques described above is to improve the signal-to-noise (S/N) ratio by segregating a non-magnetic oxide into a grain boundary to isolate a magnetic particle magnetically. In order to allow a high S/N ratio and thermal stability to be compatible, however, the magnetic anisotropy of magnetic particles needs to be increased. In this case, there has been a problem that the coercive force becomes too high, which makes it difficult for a single magnetic pole head to perform recording.
In order to solve the problem, a structure in which a ferromagnetic alloy layer not containing an oxide is laminated on a recording layer having a granular structure, in which an oxide is segregated into a grain boundary, is proposed. For example, Japanese Patent Publication No. 2004-310910 (“Patent Document 2”) discloses a configuration of a perpendicular magnetic recording layer including a layer that contains Co as a main component, and Cr, and does not contain an oxide, and a layer that contains Co as a main component, Pt and an oxide. As compared with the recording layer having the granular structure, the ferromagnetic layer which does not contain an oxide has a high magnetic coupling force between magnetic particles within the ferromagnetic layer, and has a property that magnetization is easily inverted by a magnetic held. By laminating the ferromagnetic layer not containing an oxide on the granular layer, the coercive force of the entire recording layer is reduced. As a result, a perpendicular magnetic recording medium improved in thermal stability, facilitated in recording by using a single magnetic pole head, and having a high S/N ratio can be realized.
Furthermore, in order to improve recording magnetic field gradient in a magnetic head that records information on a perpendicular magnetic recording medium, there has been proposed a magnetic head having a structure in which a magnetic shield is provided on at least a trailing side of a main magnetic pole in the track direction thereof with a non-magnetic gap layer interposed therebetween, in a structure of a known single magnetic pole type head. Hereinafter, the magnetic shield is called a trailing shield and the recording head provided with a trailing shield is called a trailing shield type recording head. For example, an example of the trailing shield type recording head is disclosed in U.S. Patent Publication No. 2002/0176214 (“Patent Document 3”) or Japanese Patent Publication No. 2005-190518 (“Patent Document 4”). In the case of the trailing shield type recording head, the recording magnetic field intensity is reduced but the recording magnetic field gradient can be increased. Accordingly, by combination of the trailing shield type recording head and the above-described perpendicular magnetic recording medium, a higher track recording density can be realized.
Moreover, an effort to improve an area recording density by increasing the track density is also made in addition to improving the linear recording density by adopting the above-described perpendicular magnetic recording method. However, as a track pitch decreases, magnetic information items recorded on adjacent tracks interfere with each other. As a result, since a magnetization transition region of a boundary region between the adjacent tracks serves as a noise, a problem that the S/N ratio is decreased becomes noticeable. In addition, writing blur (fringe) caused by the distribution of a magnetic field generated from a recording head becomes noticeable in a recording track end, which causes a problem in that magnetic information of adjacent tracks is eliminated.
A discrete track technique has been drawing attention as a method of solving the problem described above. In the case of a discrete track medium is used, it is possible to physically separate adjacent recording tracks from each other to suppress writing blur at the time of recording and interference between adjacent signals at the time of reproduction, thereby significantly increasing the track density. Discrete track media can be largely divided into two types One is a discrete track medium in which a step difference is provided on a back surface of a magnetic member which records information by performing irregularity processing on a substrate such that a recording track portion which magnetically records information becomes a protruding portion and a guard band portion provided between adjacent recording track portions becomes a recessed portion. Another one is a discrete track medium in Which a part or all of a magnetic member of a guard band portion is cut to make recording into the guard band portion impossible. As a former technique, for example, Japanese Patent Publication No. 2000-195042 (“Patent Document 5”) discloses a method of manufacturing a magnetic recording medium, the method comprising the steps of: forming concentric circular irregularities on a substrate by using an etching method; forming sequentially an underlayer, a magnetic layer, a first non-magnetic layer, and a second non-magnetic layer made of Cr, Co—Cr—Pt—Ta alloy, Cr, and SiO2 respectively; and polishing a surface of the second non-magnetic layer having irregularities to make smooth until Cr is exposed. As a latter technique, for example, Japanese Patent Publication No. 9-97419 (“Patent Document 6”) discloses a magnetic disk medium comprising a recording track portion, which magnetically records information formed of a magnetic member, and a guard band portion formed of a non-magnetic material between adjacent recording track portion, wherein, in a lower region of the guard band portion, a magnetic member is not provided or a magnetic member different in thickness from that of the recording track portion is provided.
In the methods of manufacturing a discrete medium disclosed in Patent Documents 5 and 6, a protective layer is formed, after a process of forming discrete tracks, by performing a process of filling the guard band portion with a non-magnetic material and a process of making a medium surface flat. These processes are indispensable in order to make a magnetic head stably float and travel above the medium surface at the time of writing and reading. In the above processes, a step difference between the recording track portion and the guard band portion is reduced by removing a superfluous layer formed above a magnetic layer of the recording track portion after the process of filling the non-magnetic material. In addition, in order to make surface roughness of the entire medium surface sufficiently small, a number of processes and an accurate control in each process are required. For example, Japanese Patent Publication No. 2006-196143 (“Patent Document 7”) discloses the method of manufacturing a discrete medium in which the surface is made flat by using etching selectivity between a mask layer, which is used for forming a discrete track, and a non-magnetic material filled in the guard band portion. However, in order to uniformly remove the mask layer remaining in the recording track portion while making the step difference, which occurs between a filling material surface of the guard band portion and a recording layer surface of the recording track portion, small on the entire surface of the medium, it is necessary to accurately control the conditions and time of the processes. For this reason, in the known method of manufacturing a discrete track medium, there was a problem that a manufacturing cost noticeably increases due to a reduction in yield caused by an increase in the number of processes and the complexity of processes. This problem occurs, in the known discrete medium, due to a structure in which a step difference occurs between a magnetic layer surface of the recording track portion and an uppermost surface of the guard band portion immediately after the process of forming discrete tracks.